How Is Sewage Water Purified?

Pre-Treatment

In order to become usable water, sewage water must go through a long series of treatments in several stages. Before any serious treatment can begin, all sewage water must go through a pre-treatment stage where as many solids are removed as possible. This includes not only floating excrement or trash, such as feminine hygiene products, but also small rocks that may drop to the bottom. Large metal screens are often used to collect the top trash, while sand and grit collectors may gather the material on the bottom.

After the pre-treatment, sewage water undergoes a primary treatment where dissolved solids are allowed to settle out of the sewage mixture. During the primary treatment, fats and greases float to the top and can be removed, while stacked collectors at the bottom collect the sludge that settles out from the mixture due to gravity. The sludge is also removed to be cleaned, burned, used as fertilizer, or thrown in a landfill, while the remaining sewage water (now only half is polluted) moves on to secondary treatment.

With all of the easily removable pollutants now gone from the sewage, the effluent is now subjected to various microorganisms that will break down the organic pollutants into less harmful substances. Most of these microorganisms are aerobic and require oxygen, hence the reason that most secondary treatment facilities have aeration to provide a constant flow of oxygen through the sewage to support the bacteria and other microorganisms that are doing the work.

While water can sometimes be passed along to the final stage after secondary treatment, a third stage of treatment is often used to remove nitrogen and phosphorous from the water. Oxidation breaks down the nitrogen into nitrates, which can then be turned into nitrous gasses that will float up and leave the water.

Post-treatment sewage water must be disinfected and deodorized before being released back into the ocean. Air scrubbers and carbon absorbers can be used to remove odors, while chlorination is used to kill the bacteria added to the water in earlier treatment stages. Once the water is cleaned in this manner, it is ready to be released back out into the world.